The present invention relates to automatic gain control and, more particularly, to methods and apparatus for generating automatic gain control signals and/or for adjusting a signal power reference level used by an automatic gain control (AGC) circuit.
Information is frequently communicated using digital signals. For typical digital communication signals a histogram of a sampled radio frequency (RF) signal may appear something similar to that of the Gaussian traditional bell shaped curve. Graph 100 of FIG. 1 is a histogram of a signal that is perfectly represented by a digital sampling system whose scaling ranges from a maximum negative (xe2x88x92) scale to a maximum positive (+) scale. This signal would have a standard deviation approximately equal to max_scale/3. In FIG. 1, the vertical axes corresponds to the occurrence rate of samples while the horizontal axis represents the digitized values of the samples which represent the signal. A signal""s power is equal to its standard deviation squared. Hence, by controlling a signal""s amplitude such that is equals a target standard deviation, one can approximately set the signal""s power level to the standard deviation squared.
In various signal processing operations, such as analog to digital (A/D) conversion, it is desirable to use the full useful dynamic range of the signal processing circuitry, e.g., A/D converter. However, allowing a signal to exceed the signal processing circuit""s useful dynamic range, can lead to undesirable consequences such as signal clipping or the introduction of other signal distortions. Accordingly, it is often desirable to control the power level of a signal being processed.
AGC circuits are used to control the power of a signal being processed. Frequently this is done by supplying a gain control signal, generated by an AGC circuit, to a tuner or other device which controls the gain of a signal. Many forms of automatic gain control circuits employ a power reference level to which an estimate of the signal""s power is compared. The AGC signal is then adjusted so that the signal""s power will approximate the power reference level. In most known systems, the power reference level is fixed, e.g., set to a fixed value at the time the system is manufactured.
Graph 102 of FIG. 1 illustrates what happens if the power reference level is set too high. Examining graph 102 reveals that a large amount of signal clipping will occur at the signal edges 103, non-linearly distorting the received signal. Graph 104 is a histogram of the same signal, illustrated in graphs 100 and 102, when controlled using an AGC with a power reference level that is set too low, e.g., small. Notice in this scenario, the full dynamic range 105 is not utilized. Although this problem is less severe than the signal clipping problem, it is does not allow an optimum usage of the available signal processing circuitry""s dynamic range.
FIG. 2 illustrates a receiver 200 which incorporates a known AGC circuit 208. As illustrated the receiver 200 comprises an antenna 202, tuner 204, analog to digital (A/D) converter 206, and an AGC circuit 208 which is coupled to additional signal processing circuitry, e.g., demodulator circuitry 210.
The known AGC circuit 208 estimates the current signal power level, e.g., by performing a simple squaring operation followed by a low pass filtering operation. If the current signal power estimate is below the fixed power reference level (PRL) 209, then the AGC signal, supplied to the tuner via line 212, is adjusted to cause the tuner 204 to apply more gain to the received signal. Conversely, if the AGC circuit""s current estimate of the signal power is too large, e.g., if it exceeds the fixed PRL, the AGC signal is modified so that the gain applied to the signal being processed is decreased.
It is well known that a communications channel can effectively change the peak to average power ratio of a received signal. This unfortunate result can be caused by linear distortions such as, e.g., multipath and/or large amounts of additive noise, and non-linear distortions such as those associated with receiver front end overloading. Under such noisy conditions, when a fixed power reference level is used, it is possible that signal processing circuitry, such as an analog to digital (A/D) converter or amplifier, will excessively clip the received signal causing additional signal distortions. When the A/D converter is located at the front end of a digital demodulator, e.g., following an analog tuner, the clipping introduced by the A/D converter represents demodulator front end noise which can lead to demodulation errors.
Unfortunately, many known AGC designs fail to provide maximum use of signal processing circuitry""s dynamic range and/or produce undesirable clipping in the presence of the types of noise discussed above. These problems with the known AGC circuits are due largely to the use of a fixed power reference level.
In view of the above discussion, it is apparent that there is a need for new and improved methods of performing automatic gain control. It is desirable that at least some new gain control methods support adjusting the power reference level used by a gain control circuit, e.g., in response to changing signal conditions. It is also desirable chat at least some new methods and apparatus capable of being implemented relatively easily using digital circuitry.